Pi type resistance capacitance filter unit



March 15, 1949.

M. COHEN ETAL I PI TYPE RESISTANCE CAPACITANCE FILTER UNII Original Filed June 20, 1946 INVENTOR MONTE C O H E N FREDERICK G. WEBBER ATTORNEY Patented Mar. 15, 1949 PI TYPE RESISTANCE CAPACITANCE FILTER UNIT Monte Cohen, Longmeadow, and Frederick G. Webber, Springfield, Mass., assignors to The F. W. Siclrles Company, Chicopee, Mass., a corporation of Massachusetts Original application June 20, 1946, Serial No.

Divided and this application March 20, 1948, Serial No. 15,998

'7 Claims.

The present invention relates to a unitary elecrical structure particularly adapted for use as an :lectrical filter unit in various electrical installaions.

Pi type resistance capacity networks are of comnon occurrence in electrical installations of all ;ypes and are of particular importance when of 1118 form in which the two capacitors are coniected in parallel across the line to form the legs )f the pi and a resistor is connected in the line :0 form the top of the pi. Such a network arrangement is particularly useful as a low pass filter. When such filters are employed in radio :ircuits, the two capacitors are normally of equal capacitance values, these values ranging from 60 micromicrofarads up, and the resistor is ordinarily of the value of 50,000 ohms. These relationships and values are here stated as typical only and the present invention may be adapted to whatever values are dictated by the requirements of proper circuit design.

In the past, these networks have been formed by suitable electrical connection between standard resistor units, usually of the cylindrical type, and standard capacitor units, usually mica or ceramic dielectric condensers.

It is the prime object of the present invention to provide a single electrical unit which contains within itself the resistive and capacitive elements necessary for the formation of a network structure of the type described.

The advantages of such a structure are manifold. A present major trend in radio receiver design is toward smaller and smaller units such as may be carried in the owners pocket. It is of extreme importance in such applications that the size of all circuit elements be reduced to their absolute minimum. Our invention comprises a combination of the resistor and capacitors which take up substantially no more space than the resistor itself. Consequently, by its employment in a radio receiver, the space, for example, taken up by two condensers is completely saved, and this with no loss of receiver performance.

Similar considerations are involved in the question of weight. Each standard capacitor has a substantial case, the weight of which, while not great in an absolute sense, is comparatively large. Fixed condensers of the wrapped foil type are of comparatively high weight. However, in the present invention, the resultant unitary structure is of substantially the same weight as that of the resistive unit incorporatedtherein and thus, for each of such units used, the weight of one or more standard capacitors is saved. This saving is so the invention is reduced to zero.

considerable as to be of importance not only in small portable receiving sets but also in more elaborate installations.

Equally, if not more important, than the above two considerations is the fact that the cost of the unitary structure of the present invention is far less than the combined costs of the standard resistive and capacitive units at present employed in the formation of networks of the type under discussion. In the present stage of the industry, a differentiation of a fraction of a cent per unit is of extreme industrial importance, and consequently the substantial diiferential in cost between the structure of the present invention and the combined units formerly employed looms large and would, even apart from the size and weight considerations detailed above, alone attest the importance of this invention.

The unitary structure of the present invention presents certain technical electrical advantages in addition to the economic and industrial advantages detailed above. Thus, the length of leads necessary to form the network is materially reduced, and in some embodiments of This is of particular importance in high frequency applications where the inductive effect of long leads presents serious electrical limitations upon circuit design. In addition, in some embodiments of the present invention, the unit is self-shielded, this being of importance in audio frequency circuits and being of increasing importance as the frequency rises. In cases where the unit is self-shielded, the employment of conventional shield cans or plates is madeunnecessary, with a consequent saving in size, weight, and expense over and above that already set forth.

To the accomplishment of the foregoing objects and advantages, the present invention resides in the unitary electrical structure as sought to be defined in the appended claims and as described in this specification, taken together with the accompanying drawings, in which:

Figs. 1 through 6 represent one embodiment of the present invention in various stages of construction;

Figs. '7 through 11 represent a second embodiment thereof; 7

Figs. 12 through 16 represent a third embodiment thereof; and

Fig. 17 is a circuit representation of a pi type network.

The present invention is here described as relating to a unitary electrical structure which,

Fig. 17, a pi type resistance capacity network comprising a pair of capacitors 2 and i connected in parallel between two electrical lines 6 and 8. In line 6 and between the capacitors 2 and 4 is connected a resistance it. In most applications it is contemplated that the line 8 in which the resistance it is not connected will represent the common ground of the electrical apparatus in which the network is employed. It will be appreciated, however, that variations in the described structure can easily be made to conform to variations in the network. It is further obvious that variations in the electrical values of the resistor and capacitors embodied in our structure may be made to correspond to the desired electrical Values dictated by circuit design.

The unitary structure of the present invention involves, as typically shown in Figs. 1 to 6, combining a dielectric layer A, a single capacitive plate B on one side thereof and a pair of separated capacitive plates C and C on the other side thereof, and in then combining therewith a resistor D in such a manner that the resistor becomes a physical part of the unitary structure adding no appreciable size to the pair of capacitors as defined by the three capacitive plates B, C and C, or, considered in another light, the capacitive elements B, C and C add appreciably no weight or size to the resistor D. This invention is susceptible of many variations all of which attain the desired advantages of low weight, small size, and inexpensiveness, and each of which present additional advantages and disadvantages relative to one another. Some of these variations are illustrated and described herein, and they will be suggestive of many variations without departing from the generic concept.

In the embodiment the construction of which is illustrated by Figs. 1 through 6, the structure has as its basic element a standard cylindrical electrical resistor l 2 of desired ohmic value. This resistor has a pair of leads M and [5 (Fig. 1). A conductive coating is applied in any suitable manner to all of the outer surfaces of the resistor l2 except for a narrow band is at the center thereof, thus defining the two separated capacitive plates C and C, or the conductive coating may first be applied over all the outer surfaces and then removed from the portion of the resistor l2 defined by the band l8 (Fig. 2). It will be noted that, since the metallic coatings extend over the ends I 9 of the resistor i 2 and contact the leads i4 and it, the two plates C and C' are thus electrically connected to the opposite ends of the resistor I2 without the necessity of any additional leads. This, as has already been pointed out, is particularly advantageous in high frequency applications.

To the structure of Fig. 2 is then applied a coating of insulating material, for example, glass, which coating not only insulates the plates C and C and the conductive coatings on the ends of the resistor :2 but also serves as the dielectric layer A (Fig. 3). A conductive coating is then applied over the outer cylindrical surface of the structure only to form the single capacitive plate B (Fig. 4). Lead 25 is then attached to the capacitive plate B as by soldering (Fig. 5) and the entire assembly may then be recoated with a finish 22 of an insulating material if desired (Fig. 6).

It will be apparent from the above structure that connected directly between the leads M and i6 is the resistor l2 and connected to the resistor 2 at either end thereof is a capacitive plate C and C, between each of which and the single capacitive plate B the dielectric layer A sets up capacitive effects. Consequently, there are in efiect two capacitors, one defined by plate B and plate C, and the other defined by plate B and plate C. The plates C and C are not in capacitive relation with one another. If lead 20 be connected to the line 8 of Fig. 17, the unitary structure will be electrically equivalent to the network of Fig. 17, the resistor l2 corresponding to the resistor H], the capacitor B-C corresponding to the capacitor 2 and the capacitor B-C' corresponding to the capacitor 3.

A further advantage of the structure just described, aside from the fact that no leads are necessary between the resistor 12 and the capacitive plates C and C, is the fact that the entire structure is electrically enveloped by the capacitive plate B which may in turn be connected to ground. Consequently, the capacitive plate B will additionally serve as an electrical shield for the network.

The variation illustrated in Figs. '7 through 11 comprises a tube 23 formed, for example, of a ceramic material with high dielectric constant (Fig. 7) which is first coated inside and out with a conductive coating (Fig. 8). This coating is then removed from the ends 23 of the tube 24 and also from a band It! at the center thereof (Fig. 9), thereby formin on the interior of the: tube M a single capacitive plate B and on the exterior thereof a pair of separated capacitive plates C and C both of which are separated from the single plate 13 by the dielectric tube 24. Leads- 30 and 32 may be attached to the plates C and C" as by soldering (Fig. 9). The interior diameter of the tube 24 after coating is sufficient to admittherein a standard cylindrical resistor [2' of the desired ohmic value which has leads M and IE In order to make contact with the inner capaci-- 40 tive plate B, a spring contact tab 34 is so shaped as to fit over a portion of the resistor I2 but to be urged resiliently up therefrom, this efiect being achieved by having a portion thereof nor mally assume a curved shape having a radius of curvature somewhat less than that of the exterior of the resistor l2. This tab is connected to a lead 36 (Fig. 10). The resistor I2 with the clip 34 therearound is slid into the coated tube 24 and the leads Hi and it are connected, as by soldering, to the leads 30 and 32. The lead 36 projects out from between the resistor i2 and the inner capacitive plate B, with which it makes contact via spring tab at (Fig. 11). The entire unit may then be coated with insulation as in Fig. 6.

This structure has the advantage that the capacitive elements thereof may be independently constructed in a simple and inexpensive manner and may subsequently be incorporated with the resistor. However, it will be noted that the shielding effect and the minimum lead length achieved by the previously described structure is not characteristic of this structure, although the lead. length is small compared with that involved in attainment of these networks by conventional methods.

The structure illustrated in Figs. 12 through 16 is basically similar to that of Figs. 7 through 11 except for the fact that the single capacitive plate B is formed on the exterior of the shell 24, thus achieving a built-in shielding eiiect at the expense of a somewhat more complicated manufacturing procedure and the employment of two instead of one spring clip 34. In this embodiment, the dielectric tube 24 is coated in two steps in stead of one. A tube 38 of rubber or similar ma- 5 terial is inserted into the interior of the tube 24 somewhat more than half-way and the remainder of the tube 24 is conductively coated (Fig. 12). The rubber tube 38 is then removed and inserted through the coated end of the ceramic tube 24 again more than half-way through the interior of the tube and the remainder of the tube is then coated. This results in a conductive coating extending over the ends 28 and the entire outer surface of the tube 24 and over separated portions of the inner surface thereof (Fig. 13) The tube 38 is removed and the conductive coating is eliminated from the ends 28 of the tube 24, the tube thus having a single capacitive plate 13 on the exterior thereof and a pair of separated capacitive plates C and C" on the interior thereof, the latter being separated from the single capacitive plate B by means of the dielectric tube 24. A standard cylindrical resistor I2 is again inserted into the coated interior of the tube 24 but since separate contact must be made with each of the capacitive plates and C, a pair of spring tabs 34, 34' are employed, each having a lead 36, 36 which is electrically connected to the leads l4 and N of the resistor [2 (Fig. After the resistor 12 and its attached spring tabs 34, 34 are inserted into the tube 24, a lead 40 is then attached to the single exterior capacitive plate B (Fig. 16) and the resultant structure may optionally be covered by an insulating material as in Fig. 6.

It will be apparent that, by the construction above described, there has been provided a simple unitary electrical structure of small size, low

weight, and low manufacturing cost, which is adapted to be sold as a complete article of manufacture and to serve as a complete electrical network, here illustrated as a pi type resistance capacity filter network. Such a single unitary structure is adapted to replace the employment, for the illustrated network arrangement, of three separate circuit elements, to wit, a resistor and two capacitors. The articles of manufacture of the present invention may be tailor-made to fit any circuit requirements and, should the units become damaged or should the circuit requirements change, they may be replaced as a unit merely by soldering the leads to their proper circuit connections.

It is apparent that many variations may be made in the specific structure here disclosed without departing from the spirit of the invention as sought to be defined in the following claims. For example, the capacitive layers, instead of being coated onto a standard cylindrical resistor as in Figs. 1 through 6, could be formed of foil and paper in the manner of the conventional fixed capacitor and the resultant foil and paper can then be wrapped around the outer surface of the resistor. Many other variations will undoubtedly suggest themselves.

This is a division from our application, Serial No. 677,961, filed June 20, 1946, entitled Electrical structure.

We claim:

1. A unitary electrical structure defining a pi type resistance capacitance filter network, said structure comprising a hollow cylindrical layer of dielectric material, a plate on. the exterior thereof, a pair of separated plates on the interior thereof, said plates being of material which is a good conductor of electricity and defining a pair of capacitors in which the separated plates are not in effective capacitive relation with each other, a cylindrical resistor housed within said 6. layer, said separated plates being adjacent the ends of said resistor, and direct electrical connections between said separated plates and the ends of said resistor, whereby said direct electrical connections may be of minimal length.

2. A unitary electrical structure defining a pi type resistance capacitance filter network, said structure comprising, in built-up sequence, a cylindrical resistor having a pair of axial leads, a pair of separated caps of conductive material thereon, a layer of dielectric material thereover, and a single layer of conductive material thereover, said caps and conductive layer defining a pair of capacitors in which said caps are not in effective capacitive relation with each other, said caps making electrical and physical contact with the leads of said resistor.

3. A unitary electrical structure defining 2. pi type resistance capacitance filter network, said structure comprising, in built up sequence, a cylindrical resistor, a pair of separated caps of conductive material thereon, a portion of said caps making direct electrical connection with the ends of said resistor, a layer of dielectric material over said separated caps, and a single layer of conductive material over said layer of dielectric material, said caps and conductive layer defining a pair of capacitors in which said caps are not in effective capacitive relation with each other, said caps making electrical and physical contact with the ends of said resistor.

4. A unitary electrical structure defining a pi type resistance capacitance filter network, said structure comprising a cylindrical resistor, 21. pair of separated plates on the cylindrical surface thereof, a layer of dielectric material over said separated plates, a single layer of conductive material over said layer of dielectric material, and a layer of conductive material on the ends of said resistor, said layer making direct electrical connection between said separated plates and the ends of said resistor, said plates being of material which is a good conductor of electricity and defining a pair of capacitors in which the separated plates are not in effective capacitive relation with each other.

5. A unitary electrical structure defining a pi type resistance capacitance filter network, said structure comprising a hollow cylindrical shell of dielectric material, a plate on one side thereof, a pair of separated plates on the other side thereof, said plates being of material which is a good conductor of electricity and defining a pair of capacitors in which the separated plates are not in effective capacitive relation with each other, a cylindrical resistor housed within said shell, and electrical connections between said resistor and said separated plates, the resistance thus be-- ing electrically interposed between said separated plates, the connection with a plate on the interior of said shell being made by a connective means disposed between said resistor and said plate, said connective means comprising a strip of resilient conductive material having a lead attached thereto and having a portion thereof bent to normally assume a circular shape of radius greater than that of the interior of said shell.

6. The unitary electrical structure of claim 5, in which the first named plate is on the exterior of said shell and the two separated plates are on the interior thereof, and the connection with said interior plates is made by a connective means disposed between said resistor and the appropriate conductive plate. said connective means comprising a strip of resilient conductive material having a lead attached thereto and. having a portion thereof bent to normally assume a: circular shape REFERENCES CITED of radius greater than that of. the interior of saidv The following references are of record. in the shell. file of this patent:

7. The unitary electrical structure of claim 5, 5 i i in which the first named plate. is on the interior UNITED STATES ATENTS of. said shelland the two separated plates are onv Number Name. te thev exterior thereof, and the connection-with said. 1,324,792 300th 16, interior plate. is made by a. connective means disr- ,671,478 Marbury May 29, 19.28. posed between said resistor and said plate, said 10 FOREIGN PATENTS; connective means. comprising av strip of resilient: P conductive material having a lead attached Number countfy Date thereto and having a portion thereof bent to: gf zgggg normally assume a circular shape of radius) 314,167 G at Brita n June- 711 9 9 reater. than that of the interior of said shell.

MONTE COHEN. FREDERICK. Gr WEBB-ER. 

